二日 is a time word from literary or formal Chinese. It has two main uses: (1) as a measure of duration meaning 'two days' (like 二日游 'two-day trip'), and (2) as an ordinal denoting 'the second day' (like 二日清晨 'the morning of the second day'). In everyday speech 'two days' is usually 两天, and 'the second day' is usually 第二天—二日 adds a classical or journalistic tone.